README incorrectly states ostruct is pre-packaged with Ruby (removed in Ruby 4.0.0)

[tomczak-mateusz]

Starting from Ruby 4.0.0, ostruct is no longer part of the standard library or the default gems.
The installation guide in README states:
The ostruct library comes pre-packaged with Ruby. No installation is necessary.

This is no longer true after the Ruby 4.0.0 release according to this warning:
warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 4.0.0.

The installation guide in README should be updated to inform about the explicit installation steps & to match guides in other gems removed from the default gems, like bigdecimal, csv or mutex_m.

[olleolleolle]

Thanks, @tomczak-mateusz! Would you be interested in formulating a PR about that?

[tomczak-mateusz]

@olleolleolle proposed fix:
#94